Masonry drilling tools generally comprise a drill head provided with a carbide cutting plate, and optionally side cutting plates and a single-start or multi-start spiral conveying helix, which adjoins the drill head. At the end of the conveying helix remote from the drill head is generally provided a clamping shank which can be releasably fixed within a tool holder of a rotary hammer. Such masonry drilling tools are generally driven by a rotary hammer which can simultaneously apply a rotational drive to the drilling tool and repeated axial impacts to the rearward end of the drilling tool.
In masonry drilling tools, the primary function of the conveying helix is to convey the drilled hole material, such as dust and chippings, broken away by the carbide cutting plate out of the hole. It is important to the performance of the drilling tool that a large volume of broken material, such as dust and chippings, is conveyed out of the drilled hole because this aids the advancement of the drilling tool into the drilled hole. Good material conveying properties also help to prevent jamming of the drilling tool within the drilled hole, which jamming can damage the drilling tool and the rotary hammer driving the drilling tool. The life of the drilling tool is also improved if it has good material conveying properties. The conveying helix also performs the function of guiding the drilling tool within the drilled hole.
The conveying helix generally comprises one or more discharge grooves or flutes which extend helically around the drilling tool. Each of the flutes is bounded on each side by a corresponding helically extending web. Each web has a conveying surface which faces in the direction of the clamping shank of the tool along which broken material is conveyed. The radially outwardly facing surface of the webs guides the drilling tool within the drilled hole.
There are a number of proposals for improving the conveying properties of the conveying helix, for example WO99/29998, which advocates the use of auxiliary webs and/or flutes in addition to the primary webs and flutes of a drilling tool.
Critical to the conveyance of broken material out of the drilled hole is the conveyance of broken material from the carbide cutting plate at the forward facing face of the drill head, where the broken material is generated to the entrances to the flutes of the conveying helices. For this purpose a plurality of substantially axially extending channels are generally formed around the drill head for transporting broken material from the cutting plate or plates to the entrances to the flutes of the conveying helices. This has been addressed, for example in EP657,617 and DE197 03 994 in which relatively high volume channels are provided for this purpose. Also, DE195 37 900 and DE200 21 710 propose improved designs of such channels.